Genetic diseases, cancers, and other conditions often result from or produce an imbalance in two corresponding chromosomes or alleles or other nucleic acid sequences. That is an amount of one sequence relative to another sequence is larger or smaller than normal. Usually, the normal ratio is an even 50/50 ratio. Down Syndrome (trisomy 21) is such a disease having an imbalance of an extra chromosome 21.
Conventional prenatal diagnostic methods of trisomy 21 involve the sampling of fetal materials by invasive procedures such as amniocentesis or chorionic villus sampling, which pose a finite risk of fetal loss. Non-invasive procedures, such as screening by ultrasonography and biochemical markers, have been used to risk-stratify pregnant women prior to definitive invasive diagnostic procedures. However, these screening methods typically measure epiphenomena that are associated with trisomy 21 instead of the core chromosomal abnormality, and thus have suboptimal diagnostic accuracy and other disadvantages, such as being highly influenced by gestational age.
The discovery of circulating cell-free fetal DNA in maternal plasma in 1997 offered new possibilities for noninvasive prenatal diagnosis (Lo, Y M D and Chiu, R W K 2007 Nat Rev Genet 8, 71-77). While this method has been readily applied to the prenatal diagnosis of sex-linked (Costa, J M et al. 2002 N Engl J Med 346, 1502) and certain single gene disorders (Lo, Y M D et al. 1998 N Engl J Med 339, 1734-1738), its application to the prenatal detection of fetal chromosomal aneuploidies has represented a considerable challenge (Lo, Y M D and Chiu, R W K 2007, supra). First, fetal nucleic acids co-exist in maternal plasma with a high background of nucleic acids of maternal origin that can often interfere with the analysis (Lo, Y M D et al. 1998 Am J Hum Genet 62, 768-775). Second, fetal nucleic acids circulate in maternal plasma predominantly in a cell-free form, making it difficult to derive dosage information of genes or chromosomes within the fetal genome.
Significant developments overcoming these challenges have recently been made (Benachi, A & Costa, J M 2007 Lancet 369, 440-442). One approach detects fetal-specific nucleic acids in the maternal plasma, thus overcoming the problem of maternal background interference (Lo, Y M D and Chiu, R W K 2007, supra). Dosage of chromosome 21 was inferred from the ratios of polymorphic alleles in the placenta-derived DNA/RNA molecules. However, this method is less accurate when samples contain lower amount of the targeted gene and can only be applied to fetuses who are heterozygous for the targeted polymorphisms, which is only a subset of the population if one polymorphism is used.
Dhallan et al (Dhallan, R, et al. 2007, supra Dhallan, R, et al. 2007 Lancet 369, 474-481) described an alternative strategy of enriching the proportion of circulating fetal DNA by adding formaldehyde to maternal plasma. The proportion of chromosome 21 sequences contributed by the fetus in maternal plasma was determined by assessing the ratio of paternally-inherited fetal-specific alleles to non-fetal-specific alleles for single nucleotide polymorphisms (SNPs) on chromosome 21. SNP ratios were similarly computed for a reference chromosome. An imbalance of fetal chromosome 21 was then inferred by detecting a statistically significant difference between the SNP ratios for chromosome 21 and those of the reference chromosome, where significant is defined using a fixed p-value of ≦0.05. To ensure high population coverage, more than 500 SNPs were targeted per chromosome. However, there have been controversies regarding the effectiveness of formaldehyde to enrich to a high proportion (Chung, G T Y, et al. 2005 Clin Chem 51, 655-658), and thus the reproducibility of the method needs to be further evaluated. Also, as each fetus and mother would be informative for a different number of SNPs for each chromosome, the power of the statistical test for SNP ratio comparison would be variable from case to case (Lo, Y M D & Chiu, R W K. 2007 Lancet 369, 1997). Furthermore, since these approaches depend on the detection of genetic polymorphisms, they are limited to fetuses heterozygous for these polymorphisms.
Using polymerase chain reaction (PCR) and DNA quantification of a chromosome 21 locus and a reference locus in amniocyte cultures obtained from trisomy 21 and euploid fetuses, Zimmermann et al (2002 Clin Chem 48, 362-363) were able to distinguish the two groups of fetuses based on the 1.5-fold increase in chromosome 21 DNA sequences in the former. Since a 2-fold difference in DNA template concentration constitutes a difference of only one threshold cycle (Ct), the discrimination of a 1.5-fold difference has been the limit of conventional real-time PCR. To achieve finer degrees of quantitative discrimination, alternative strategies are needed. Accordingly, some embodiments of the present invention use digital PCR (Vogelstein, B et al. 1999 Proc Natl Acad Sci USA 96, 9236-9241) for this purpose.
Digital PCR has been developed for the detection of allelic ratio skewing in nucleic acid samples (Chang, H W et al. 2002 J Natl Cancer Inst 94, 1697-1703). Clinically, it has been shown to be useful for the detection of loss of heterozygosity (LOH) in tumor DNA samples (Zhou, W. et al. 2002 Lancet 359, 219-225). For the analysis of digital PCR results, sequential probability ratio testing (SPRT) has been adopted by previous studies to classify the experimental results as being suggestive of the presence of LOH in a sample or not (El Karoui at al. 2006 Stat Med 25, 3124-3133). In methods used in the previous studies, the cutoff value to determine LOH used a fixed reference ratio of the two alleles in the DNA of 2/3. As the amount, proportion, and concentration of fetal nucleic acids in maternal plasma are variable, these methods are not suitable for detecting trisomy 21 using fetal nucleic acids in a background of maternal nucleic acids in maternal plasma.
It is desirable to have a noninvasive test for fetal trisomy 21 (and other imbalances) detection based on circulating fetal nucleic acid analysis, especially one that is independent of the use of genetic polymorphisms and/or of fetal-specific markers. It is also desirable to have accurate determination of cutoff values and counting of sequences, which can reduce the number of wells of data and/or the amount of maternal plasma nucleic acid molecules necessary for accuracy, thus providing increased efficiency and cost-effectiveness. It is also desirable that noninvasive tests have high sensitivity and specificity to minimize false diagnoses.
Another application for fetal DNA detection in maternal plasma is for the prenatal diagnosis of single gene disorders such as beta-thalassemia. However, as fetal DNA only constitutes a minor fraction of DNA in maternal plasma, it is thought that this approach can only detect a mutation that a fetus has inherited from its father, but which is absent from the mother. Examples of this include the 4 bp deletion in codon 41/42 of the beta-globin gene causing beta-thalassemia (Chiu RWK et al. 2002 Lancet, 360, 998-1000) and the Q890X mutation of the cystic fibrosis transmembrance conductance regulator gene causing cystic fibrosis (Gonzalez-Gonzalez et al 2002 Prenat Diagn, 22, 946-8). However, as both beta-thalassemia and cystic fibrosis are autosomal recessive conditions, in which the fetus would need to inherit a mutation from each parent before the disease would manifest itself, the detection of merely the paternally-inherited mutation would only increase the risk of having the fetus having the disease from 25% to 50%. Diagnostically this is not ideal. Thus, the main diagnostic application of the existing approach would be for the scenario when no paternally-inherited fetal mutation can be detected in maternal plasma, when the fetus can then be excluded from having the homozygous disease state. However, diagnostically, this approach has the disadvantage that the conclusion is made based on the negative detection of the paternal mutation. Thus, an approach which would allow the complete fetal genotype (be it homozygous normal, homozygous mutant, or heterozygous) to be determined from maternal plasma, without the above limitation, would be very desirable.